1. Field of the Invention
This invention relates to x-ray scanners and more particularly to a method for eliminating parasitic noise of microphonic origin in a scanner of this type. The invention is also concerned with a system for carrying out said method.
2. Description of the Prior Art
In order to examine a patient, x-ray devices known as scanners are coming into increasingly frequent use for producing images of cross-sections of the patient's body. These devices are based on the physical phenomenon of absorption of x-rays by the human body. This absorption is directly related to the distance x of travel of the x-rays within the body in accordance with the formula: EQU I=I.sub.o e.sup.-bx
where:
I.sub.o is the intensity of radiation which passes into the human body, PA1 I is the intensity of radiation which passes out of the human body, PA1 b is a coefficient of attenuation which depends on the body through which the radiation passes. PA1 computation per channel of the mean value V.sub.p of the attenuation signals corresponding to the p first consecutive views V.sub.1 to V.sub.p PA1 subtraction per channel of said mean value V.sub.p from the signal of the view V.sub.1 so as to obtain a signal H.sub.1 corresponding essentially to the high-frequency components of the signal of the view V.sub.1 and to the low-frequency components of the useful signal, PA1 filtering of said signal H.sub.1 so as to eliminate the low-frequency components and to obtain a signal H'.sub.1, PA1 subtraction per channel of said signal H'.sub.1 from the signal of the view V.sub.1 so as to obtain a signal V'.sub.1 which is free of high-frequency components, PA1 subtraction per channel of said signal V'.sub.1 from the signal of the view V.sub.2 so as to obtain a signal H.sub.2 corresponding to the high-frequency components of the signal of the view V.sub.2 and to the low-frequency components of the useful signal, PA1 subtraction per channel of said signal H.sub.2 from the signal of the view V.sub.2 so as to obtain a signal V'.sub.2 which is free of high-frequency components, PA1 sequential reiteration of the two last-mentioned operations on the signals of the views V.sub.3 to V.sub.m and V'.sub.2 to V'.sub.m-1. PA1 a circuit for computing the mean value V.sub.p of the signals on p views V.sub.1 to V.sub.p PA1 a subtraction circuit for subtracting the mean value V.sub.p from the signal of the view V.sub.1 and obtaining the signal H.sub.1, PA1 a filter for eliminating the low-frequency components in the signal H.sub.1 and obtaining a signal H'.sub.1, PA1 a second subtraction circuit for subtracting the signal H'.sub.1 from the signal of the view V.sub.1 and obtaining the signal V'.sub.1, PA1 a sequential circuit for successively processing the signals of the views V.sub.2 to V.sub.m so as to subtract therefrom the signals V'.sub.1 to V'.sub.m-1 respectively and then to subtract the result H.sub.2 to H.sub.m of this subtraction from the signals of the views V.sub.2 to V.sub.m, PA1 a second memory for recording the signals V'.sub.1 to V'.sub.m delivered by the sequential circuit.
In a logarithmic measurement scale, the attenuation I/I.sub.o is equal to bx or in other words proportional to the distance x.
As shown in FIG. 1 of the accompanying drawings, a scanner is essentially constituted by an x-ray source 10 associated with a detection device 11, these two elements being disposed in a fixed geometrical relationship to each other in order that the body to be examined may be positioned between them. Furthermore, they are supported by a structure (not shown) which is capable of rotating about the body to be examined so as to irradiate the body at different angles. The x-ray source which is controlled by a device 13 emits x-rays in an angular sector which has a sufficient width to illuminate the entire cross-section of the body. The detection device 11 has the shape of an annular sector, the length of which is adapted to the width of the x-ray beam and is constituted by a large number of elementary detectors 12 in juxtaposed relation.
In order to obtain an image of the cross-section of the human body through which the x-ray beam passes, the structure which supports the source 10 and the detection device 11 is caused to rotate around the body and the output signals of the elementary detectors 12 are measured, then suitably processed in accordance with known methods in order to obtain a representative image of the cross-section. For this processing operation, the elementary detectors 12 (also known as channels) are connected to an electronic device 14 which first carries out a computation of the logarithm of the signals received so as to obtain a signal whose amplitude is proportional to the attenuation of the x-rays.
The signal which is delivered by an elementary detector is constituted by a useful signal mixed with noise. The noise arises from a number of causes such as, for example, the detector itself, vibrations of the scanner, and so on. This results in deterioration of the signal-to-noise ratio and in the appearance of artifacts or defects on the image produced by the electronic device 14 (FIG. 1) when these different noises are not sufficiently reduced by better mechanical construction of the scanner or suitable electronic processing of the useful signal. These mechanical and electronic solutions are difficult to apply in practice and lead to a substantial increase in cost price.
The method in accordance with the invention is based on the observation that, at the time of examination of the patient's body by the scanner, the signals of the channels must not exhibit differences having high-frequency components from one view to the next. If such differences exist, they must be detected in order to permit the possibility of subtracting them from the initial signal and thus to obtain a signal which is free from these high-frequency components.